The present invention generally relates to the field of brushless alternators and more particularly to brushless alternators which comprise stationary field and stator assemblies having an air gap therebetween which is radial with respect to the axis of rotation of a drive shaft that rotates magnetic fingers in the radial air gap to vary the magnetic coupling between the stator and rotor assemblies.
Brushless alternators are distinct from brush type alternators in that brush type alternators require stationary electrical contacts (brushes) fixed to the housing to provide a continuous connection to a rotating electrical assembly. Typically the rotating assembly comprises a rotating magnetic field coil assembly of the alternator, and the continuous electrical connection is provided by a friction contact to a rotating slip ring. In brushless alternators, the configuration of the alternator is such that there is no need to provide an electrical connection between stationary electrical contacts and rotating electrical assemblies.
Two general types of brushless alternators exist. In one such type a rotating magnetic field assembly is provided and it receives electrical energization from a stationary energy source, but this is not provided by a physical electrical connection to the energy source provided by brushes, but by magnetic coupling to the source which is accomplished through the utilization of an electrical transformer type structure. An alternator of this type is illustrated in U.S. Pat. No. 3,614,593, and voltage regulators for alternators of this type are illustrated in U.S. Pat. Nos. 3,617,857 and 3,629,689, all of these patents assigned to the same assignee as the present invention.
The present invention does not deal with alternators having a rotating field coil assembly, but instead deals with brushless alternators having stationary both stator and field coil assemblies but providing rotated magnetic fingers which vary the magnetic coupling between the field and stator assemblies. The fingers are rotated about a shaft axis and pass through a radial cylindrical air gap located between the stator and field coil assemblies. In such brushless alternator assemblies which have been previously provided, typically the configuration of these alternators has not been optimized to provide a maximum output current for a fixed amount of alternator volume. In other words the volumetric efficiency of such prior brushless alternators has not been substantial. In addition, such prior brushless alternators typically require a substantial number of relatively costly magnetic circuit elements and/or they undesirably extend either the axial length or the diameter dimensions of the alternator due to their structural configuration. Also, the configuration of these prior brushless alternators typically limits the amount of volume internal to the alternator which can be used for grease reservoirs for the rotatable shaft bearings, since if a larger grease reservoir were provided for insuring a longer lifetime of the alternator bearings either the size of the alternator would be undesirably increased or the electrical performance of the alternator would be decreased by the removal of a substantial amount of magnetic circuit material in critical magnetic circuit areas.
In summary, prior brushless alternators utilizing stationary field and stator assemblies; (1) did not provide high volumetric efficiencies, (2) did not implement a compact form factor (axial length versus diameter) for the alternator due to their configuration, (3) typically required a large number of magnetic circuit members, (4) were costly due to the use of additional magnetic circuit members, and (5) did not readily permit the utilization of large alternator shaft bearing grease reservoirs without either compromising the electrical output of the alternator or undesirably increasing the alternator dimensions.
The term "flux air gap" as used herein refers to an air space between magnetic circuit components across which a substantial magnetic flux passes due to the relatively low reluctance of this air gap as compared to other air gaps therebetween wherein the reluctance of the gap is a function of the air spacing between the magnetic circuit components and the surface areas of these components which face each other across the air gap. The term "primary" flux air gap as used herein refers to the flux gap between the stationary stator core and rotated magnetic circuit members, while the term "secondary" flux air gap refers to the flux air gap between the stationary field core and rotated magnetic circuit members. The terms "axial" and "radial" as used herein refer to structural orientations with respect to the axis of rotation of the rotor which coincides with the rotational axis of the alternator shaft.
Typically, prior art brushless alternators having stationary field coil and stator assemblies have two primary cylindrical radial flux air gaps between the stator and magnetic fingers which are rotated in a cylindrical radial gap between the field and stator cores by a shaft. These primary flux air gaps are axially spaced apart, but adjacent to each other and located in the radial gap between the stator and field assemblies at substantially the same radial position with respect to the shaft. Typically these alternators utilize two secondary radial flux air gaps between the field coil core and extensions of the rotated magnetic fingers.
In some alternators of the preceding type at least one of these secondary radial flux air gaps is positioned radially spaced apart from the cylindrical gap between the field and stator assemblies and positioned axially with respect to the rotatable shaft in substantially the same, axial position as the primary radial flux air gaps thus leading to an undesirable increase in the alternator diameter due to the radial stacking of the primary and secondary flux gaps.
Some prior art brushless alternators of the preceding description which utilize two radial primary and two radial secondary flux air gaps position both of the secondary flux air gaps axially apart with respect to the shaft axis and axially spaced on each side the primary flux gaps. However, this configuration leads to a substantial increase in the axial length of the alternator along the rotated shaft axis, and this is also undesirable.
In one known prior brushless alternator configuration utilizing stationary field and stator coil assemblies, a hybrid axial and radial flux air gap is provided for one secondary flux air gap by a diagonal extension (as viewed in a cross section of the alternator along the shaft axis) of the field coil core forming a diagonal flux air gap with a mating diagonal extension of a magnetic member coupled to the rotating magnetic fingers. However, in this configuration the diagonal flux gap is provided in the same axial position as the primary flux gaps and the stationary field coil thus requiring a substantial increase in the diameter of the alternator to insure that magnetic saturation does not occur either in the diagonal extension of the field core or in the corresponding diagonally magnetic finger extension which mates with the field core diagonal extension. Due to the location of the diagonal flux gap, a relatively high reluctance flux gap is provided since only a relatively small cross sectional area is available for this gap.
The diagonal gap configuration discussed above apparently does not appreciate that its configuration can lead to saturation of its magnetic circuit elements which will inherently limit the output of the alternator. This also appears to be the case with other brushless alternator configurations with provide designs wherein the field core can be saturated at low alternator output levels. Also, the diagonal gap configuration provides a shaft bearing which is also axially positioned along the shaft axis in the same axial position as the diagonal flux gap and the field coil. This configuration, if the size of the alternator is not to be increased, requires a further reduction in the size of the diagonal field core extension which conducts flux, thus causing magnetic saturation of this element at even lower ampere turns of the field coil. This will limit the alternator output. This is caused by the effective radial stacking of the bearing, the field core diagonal extension, the magnetic finger diagonal extension, the field coil and the primary flux air gap, all of which are located in substantially the same axial position.